A known method of producing a metal casting, generally termed gravity casting, involves supplying metal to a mould cavity via a ladle or similar device through a running system with the metal entry point situated at or above the top of the mould cavity. In this casting method all the metal entering the mould cavity is subjected to some turbulence. Hence turbulence associated defects can often be a problem in castings produced by this method. These defects generally take the form of oxide inclusions and entrapped gas porosity, but may also include excessive mould erosion and the development of hot spots in the moulds.
The above disadvantage of gravity casting can be overcome, at least to some extent, by filling the mould through one or more in-gates below the top of the mould cavity from a source below the mould via a mechanism which allows complete filling of the mould. By doing this the force of gravity acts against the general upward flow of metal, helping to eliminate any turbulence caused by free falling liquid metal.
This method is generally termed low pressure casting and one known form of this method involves filling a metal mould via in-gates at the bottom of the mould cavity from a liquid metal source located beneath the mould. The metal source is usually contained in a pressure vessel and by increasing the pressure in the vessel, metal is pumped into the mould. A disadvantage of this method of casting is that the direction of solidification, which must always be towards a source of liquid feed metal, is from the coldest liquid metal at the top of the mould towards the hot test metal at the bottom. Natural convection within the mould, however, attempts to move the hot metal to the top of the mould and hence opposes the direction of solidification in the mould. This reduces directional solidification within the mould and problems can often be encountered in obtaining castings free from shrinkage porosity which occurs when sections of metal solidify within the mould and are not fed by the supply of liquid metal.
One method of overcoming the natural convection within the metal moulds and forcing solidification towards the feed metal at the bottom of the mould is to use channels within the mould which carry some form of cooling medium. These cooling channels are generally carried within the upper portion of the mould and force solidification to proceed down towards the feed metal at the bottom of the mould.
A major disadvantage of low pressure casting, however, is that the mould must stay connected to the metal source for a sufficient time for the casting in the mould to solidify or at least to become self-supporting. Therefore, for high rates of productivity, multiple casting stations and sets of expensive moulds are necessary.
A second known variation of the low pressure casting method involves filling a sand mould via in-gates at the bottom of the mould from a metal source located beneath the bottom of the mould. In a further variation of this method a small secondary metal source can be incorporated in the mould cavity itself. By using light weight disposable sand moulds and incorporating the secondary metal source, the mould can be rotated and then disconnected from the primary metal source. The casting is allowed to solidify elsewhere whilst being fed from the secondary metal source. This method allows the casting operation to take place independent of the time taken for the casting to solidify, thus greatly improving the productivity of the casting station.
A major disadvantage of simple sand moulds, however, is the low thermal gradients that are formed within the liquid metal in the moulds, especially when compared with those formed in metal moulds. With low thermal gradients, large areas of only partially solidified metal can develop ahead of the advancing solidification front and it is through these areas that liquid metal must be fed. This can often prove impossible and dispersed shrinkage porosity can result. The extent of this partially solidified zone is also alloy dependent and with lower thermal gradients, there will be a smaller range of alloys that can be easily cast to produce a sound component.
Other disadvantages associated with conventional sand mould casting include the slow solidification rates that are associated with sand casting resulting in coarse microstructures, especially when compared with the structures obtained in metal moulds. The microstructure of a casting is extremely important when considering mechanical properties, with finer microstructures leading to improvements in the entire range of mechanical properties.
Furthermore, the design of the feeding system for providing metal to the mould during solidification is, in part, dependent on the solidification time of the article being cast, since the feeding system must freeze last in the solidification process. If solidification times for the article being cast can be significantly reduced, the volume of metal required in the feeding system can be decreased correspondingly with potentially significant increases in casting yields.
In conventional sand moulds, thermally conductive inserts, called "chills", are often used. However, such chills cannot provide the benefits of the present invention. Chills provide only local and temporary directional solidification as they are placed in discrete sections of the mould and only provide heat extraction until the chill approaches the temperature of the solidifying metal. The mould combination and the resultant prolonged heat extraction achieved by the present invention have not been used before and represent an innovative and significant advance in mould design for the casting of aluminium alloys and other metals.